This invention relates to systems including cranes for use in connection with furnaces for baking or calcining carbon bodies, which bodies are to be used in furnaces for electro-thermal processes or in cells for the electrolytic reduction of metal oxides to metals. More specifically, the invention relates to a structure for drawing off, by suction, coke from a so-called ring-chamber baking furnace.
In the following, by way of example, the invention will be described as applied in connection with the production of carbon bodies for the electrolytic reduction of alumina to aluminum, but it will be understood that the apparatus can be applied to the production of carbon bodies for other processes as mentioned above.
The carbon bodies which are used in cells for producing aluminum are made from a mixture, or paste, consisting of calcined antracite, petroleum coke and pitch, in proportions varying with the raw materials and the purpose which the carbon body is to serve. The pitch acts as a binding agent.
Carbon bodies for use in furnaces for electro-thermal processes, in linings and/or as electrodes, are made from other mixtures, but along the same principles.
As it comes out of the mixing machine, this paste is firm at room temperature, becomes softer around 100.degree.-150.degree. C., and at higher temperatures the volatile components will be driven off and the binding agent carbonized, whereby the paste becomes stiff and hard. This thermal treatment is referred to as baking. The baking of, for example, an anode in an electrolytic reduction cell can be effected by the heat generated by the process, but today it is normally preferred to perform this baking in separate furnaces, as this allows better control over the baking process and thus the properties of the finished product.
The carbon bodies which are placed into a furnace of this type for baking are often referred to as "green carbons" (in which "green" indicates that they are "unripe"). The green carbons are produced by pressing or vibrating, and can be of considerable size. Cathode carbons, for electrolytic reduction cells for producing aluminum, for example, can measure 700.times.900.times.4000 mm, and weigh some 4 tonnes. It is clear that when such a body is heated up and passes through a temperature zone in which it becomes soft, it will become deformed unless special steps are taken. For this reason, the green carbons are placed in high pits in the chambers, which pits are built of refractories, and the space between the carbons and the pit walls are filled with coke breeze, which will also protect the green carbons against combustion.
A certain number of pits form a chamber, and a number of chambers are built together to form a ring furnace with or without a separate lid over each chamber. The following description is based on a ring-chamber furnace with lids, but the equipment can also be used in connection with ring-chamber furnaces without lids.
Hot combustion gases are led into a chamber. They flow through hollow refractory pit walls and from there on to the next chamber. The firing zone moves through the entire furnace, thereby making the best possible use of the heat. The baking of the green carbons is completed successively, and the chambers containing the finished carbons must be allowed to cool for some time before the packing coke can be removed and the finished carbons lifted out. During baking, the temperature reaches 1280.degree. C., and the entire cycle takes about three weeks.
Ring-chamber furnaces are large. Cranes over such furnaces can, for example, have a reach of over 30 meters, and the plant is expensive. It is therefore necessary to make effective use of expedient aids.
The physical work involved in operating a ring-chamber furnace refers mainly to the emptying of the chambers at the end of the cycle, and filling the chambers with green carbons and packing coke for the next cycle.
The emptying of the chambers is started by lifting off the lid of the individual chamber, and then drawing off the packing coke by suction in order to gain access to the baked carbon.
The drawing-off of the packing coke is usually performed by lowering a suction tube down into the packing coke. This tube is normally suspended from a crane and is telescopically connected with a silo and a suction connection (fitting) on the crane bridge. The tube is lowered down into the loose coke mainly by its own weight, and is often guided manually by handles welded on to the tube.
In a fairly large baking furnace, with an annual capacity of 36,000 tonnes of calcined carbon bodies, it is necessary to draw off daily some 33 tonnes of packing coke which has a temperature of some 250.degree. C.
A previous method of doing this was based on a vacuum line installed around the furnace to which a flexible hose could be connected. This hose was connected to a transportable container, which in turn was connected to a suction tube which the operator lowered down into the pit to be emptied. The container had a capacity of some 5 tonnes of coke, and when it was full, it was taken away by crane for emptying.